The present invention relates to products made of absorbent paper and more particularly made of cellulose wadding, for sanitary or household use and is aimed at a sheet consisting of at least two plies, one of which is embossed, intended for producing toilet paper essentially, but also tissues, serviettes or kitchen roll.
In the sanitary or household paper industry, use is made, for producing these products, of an absorbent paper, generally crxc3xaaped, of low grammage known as cellulose wadding or tissue. The capacity of the structure to lengthen, which capacity is given, for example, by crxc3xaaping, is put to good use to allow the sheet to be embossed, that is to say to deform it in places permanently and obtain protrusions on one face corresponding to hollows on the other face.
Specifically, the tendency over the last few years, as regards hygiene products, has been to make them softer and gentler by working with their thickness and strength characteristics, particularly using embossing. Embossing also makes it possible to improve the visual appearance of the product. The embossing operation is performed either on paper with a high moisture content, that is to say in the wet part of the paper-making machine, or on paper with a low moisture content, that is to say during converting. The present invention relates to paper converted in the dry part.
The most commonly used embossing patterns consist of a geometrical repeat of elemental protrusions of small area or transverse top, that is to say with a small area at the top, and with a simple geometric shape. An example is described in patent U.S. Pat. No. 3,414,459 which relates to a sheet with two or three plies, that is to say consisting of a number of elemental sheets, known as plies, connected or bonded together. The plies are embossed with a protrusion repeat and a protrusion height that suit the production of water-absorbing products such as kitchen roll, the number of protrusions ranging from 5 to 30 per cm2. In addition, the Applicant Company has developed, for toilet paper in particular, patterns with a higher number of elements, ranging from 30 to 80 per cm2. In this case, because of their number, the elements form protrusions of a shorter height and of necessity have an elemental area at the top which is also very small, less than 1 mm2. In these last embodiments, an appearance is obtained which mimics that of a woven product. An example is described in patent EP 0 426 548. This type of embossing affects mainly the characteristics associated with the thickness of the sheet, on the one hand, and with its rigidity and strength, on the other hand. These patterns make it possible to reach a good compromise between the desired improvements in the characteristics when the semi-finished product is being converted into a finished product, and industrial operating conditions; in particular, they allow sufficient embossing intensity to be employed. However, they have a limited visual attraction.
Other products made of absorbent paper consist of two or more plies but are obtained according to a different association technique. On the one hand, a single or double ply is embossed and, on the other hand, a second ply, also single or double, is embossed, but using two separate embossing devices. The first ply and the second ply are embossed with patterns in relief which consist of protrusions. The pattern density of necessity remains low: less than 20 protrusions/cm2 for this type of product. What happens is that the two plies are associated using a marrying roll so that the distal surfaces of the protrusions of the two superposed plies are arranged facing areas located between two protrusions in the plane of the second ply. This technique is also known as the xe2x80x9cnestedxe2x80x9d process. With a structure of this type, the objective is to obtain thick products with a good handle. However, and especially for application to toilet paper, gentleness is not optimized because of the relatively coarse embossing. Furthermore, it is necessary for the embossing to be significant in order to obtain the desired great thickness.
In both double-sheet embodiments the two plies are embossed. While embossing gives each of the plies thickness, it does, however, lead to an appreciable reduction in its rupture strength. What happens is that the mechanical work done on the sheet is accompanied by a relaxation of the inter-fiber connections in the embossed regions.
The first objective of the invention is therefore to produce a double sheet, the rupture strength of which is improved by comparison with an embossed double sheet of the same thickness.
Conversely, by increasing the embossing pressure, the thickness of the sheet and/or the quality of the marking of the embossed pattern is increased, but this is limited by the need not to introduce an excessive drop in rupture strength.
The second objective of the invention is to produce a double sheet whose thickness and/or in which the visibility of the embossed pattern is improved by comparison with an embossed double sheet of the prior art while at the same time having good rupture strength.
According to the invention, a sheet satisfying one or other of the two objectives mentioned hereinabove, consisting of a ply made of crxc3xaaped cellulose wadding of grammage 10 to 40 g/m2 and embossed with patterns in relief consisting at least partly of discrete protrusions facing toward the inside of the sheet, and at least one unembossed ply, is characterized in that the embossed ply has, over at least a portion of its area, at least 30 protrusions per cm2, the area at the top of which is less than 1 mm2 and preferably less than 0.7 mm2, the two plies having different grammages and/or different fiber compositions.
In particular, the grammages of the plies differ by more than 5%. As a preference, they differ by 5 to 30%.
The Applicant Company has found, surprisingly, that a double sheet thus characterized has the same thickness as a double sheet produced with two plies embossed in the same way as the embossed ply of the invention and assembled in the point-point position.
Furthermore, the Applicant Company has found, all other parameters being equal, that it is possible to a certain extent to reduce the grammage of the embossed ply while at the same time maintaining the resistance of the sheet to tearing. This is the strength measured in the transverse direction of the sheet. This combination thus allows an economy of material without sacrificing the robustness of the product or even its thickness. For example, it has been found that a sheet consisting of an embossed ply of 21 g/m2 and an unembossed ply of 23 g/m2 has the same strength in the transverse direction as a sheet consisting of the two 23 g/m2 plies one of which had been embossed under the same conditions, that is to say with the same amount of imprint, as the previous 21 g/m2 ply.
It is possible to produce other combinations within the context of the invention.
Thus, it is possible to keep the same overall grammage and the same thickness but go for a stronger sheet. To this end, according to one embodiment according to the invention, an unembossed ply is chosen whose resistance to tearing is the same as that of the embossed ply but whose grammage is slightly higher. The increase in weight is compensated for by using a lighter embossed ply which is embossed in such a way as to obtain the same thickness as the previous sheet. The loss in strength of the entity resulting from the embossing of the lighter ply is lower thanks to the use of a heavier unembossed ply, the overall grammage itself remaining unchanged.
According to another embodiment, the embossed ply has a higher resistance to tearing. It is, for example, a paper, the fiber composition of which contains more long fibers. Indeed, it has been found that losses in strength are lower if the stronger ply is the embossed one.
According to another embodiment, the embossed ply has a greater grammage than the unembossed ply. By comparison with a product in which both plies are identical and have the same total grammage the physical characteristics are equal. However, it is found that there is an improvement in the visibility of the embossed pattern and in the quality of the marking.
It will be understood that it is thus possible, by applying the same teachings, to obtain a thicker sheet by embossing the embossed sheet more heavily while at the same time slightly increasing the grammage of the unembossed ply.
The properties concerned with the thickness and the strength have been observed only for pattern densities at least equal to 30 bobbles per cm2 on portions of area covering at least 30% of the total area.
The higher the number of protrusions locally, the lower the area at the top of these protrusions will be. It is preferably less than 0.7 mm2 for 30 protrusions per cm2 and preferably less than or equal to 0.4 mm2 for 50 protrusions per cm2.
In this technical field, xe2x80x9cthicknessxe2x80x9d means the thickness measured on a stack of a certain number of sheets (for example 12 plies) to which slight pressure (2 kPa for example) is applied. It follows that the thickness takes into consideration a certain resistance that the structure has to crushing. Thus, without wishing to be tied to an explanation, maintaining the thickness of the structure of the invention results from the better resistance to crushing of a ply which has been embossed with a high pattern density.
According to another characteristic, the product according to the invention has at least 30 protrusions per cm2 over at least 50% of the total area.
According to another characteristic, the number of protrusions is at least 50 per cm2.
According to another characteristic, the product has a second embossed pattern between said portions of area. Advantageously, this pattern may be of linear type, that is to say may consist of protrusions whose area at the top is of linear shape and depicts, for example, the outline of a flower. More specifically, the first and the second pattern may be at different levels. In other words, their respective tops may be at different heights with respect to the plane of the ply.